In towed marine seismic exploration, a hydrophone array is typically towed behind a marine vessel near the sea surface. The hydrophones are mounted to multiple sensor cables, commonly referred to as streamers. The streamers serve as platforms or carriers for the hydrophones, which are distributed along the length of each streamer in the array.
A set of seismic sources, also towed near the sea surface, are operated to periodically emit acoustic energy. The acoustic energy of interest propagates downward through the seawater (or other water column), penetrates the ocean floor, reflects from the subsea strata and other underlying structures, and returns upward through the water column to the hydrophone array.
The reflected seismic energy (or acoustic wave energy) arrives at receiver points in the towed hydrophone array. The array includes many such receiver points, distributed along each of the streamer cables, with sensors configured to generate data records characterizing the upward-traveling acoustic wavelets (or seismic waves) received from the subsurface structures beneath the seabed, at each of the receiver points. The hydrophone data recordings are later processed to generate seismic images of the underlying structure
In the field of subsea seismic exploration, there has recently been a demand for seismic equipment operators to conduct their surveys with the seismic equipment submerged below the depths at which most seismic surveys have been conducted in the past. These new, deeper operating targets can now lie well below the depth of the surface-referenced equipment (i.e., the vessel and the paravanes) that is used to tow and laterally spread the seismic sensors.
Typical marine depressors for maintaining equipment at a substantially constant submerged depth tend to be fairly small with very poor aspect ratios thus resulting in low lift. Aspect ratio is defined as the span of the depressor divided by its chord line length. Wings with high aspect ratios generate high downward lift forces for minimal drag (such that lift-to-drag ratios as high as 10:1 or more are possible), whereas wings with aspect ratios as low as 1 or 2 (i.e., where span and chord are roughly the same scale) will typically have lift-to-drag ratios as low as 2:1, or even lower. Conventional depressors (see, e.g., Hydro Force Technologies AS “HFT Catfish 100” (<http://www.hft.no/catfish/>), or YSI Incorporated “V-Fin” (<https://www.ysi.com/File%20Library/Documents/Specification%20Sheets/E72-Standard-V-fins.pdf>) often also provide payload bays which can be used to hold additional ballast to supplement the downforce generated by the depressor.
The problem with using deadweight to generate downforce is that it does not scale with tow speed—it provides a constant downforce regardless of how fast the depressor is moving through the water. This is often disadvantageous for those applications where a range of operational speeds is expected, with the requirement that the towed equipment maintain a stable depth over that speed range. Consequently, there is no easy, economical, or ideal way to submerge and operate seismic equipment, such as towed streamer cables, at the desired lower depths.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the invention as defined in the claims is to be bound.